Bump actions/checkout to v5

v4 ran on Node 20, which GitHub will force off the runners from
June 2026 onward. v5 runs on Node 24 and removes the deprecation
warning surfaced on every job.

Author: webrek

.github/workflows/ci.yml

@@ -37,7 +37,7 @@ jobs:
       MONGO_DB_DATABASE: permission_test
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
 
       - name: Setup PHP
         uses: shivammathur/setup-php@v2